The present invention provides a method for transporting elongate members. In particular, the method of the present invention is especially suited to transporting oil industry equipment such as risers or riser towers through water.
Equipment such as risers and riser towers are typically used in the oil and gas industry for deep water applications and consequently may be over one kilometre long and can require transportation to offshore locations, before subsea installation.
One method of transportation includes providing the equipment in sections, transporting the sections on board a vessel to the region in which they are to be deployed, assembling the sections on board a specially built vessel and deploying the equipment to the required location. Assembling the equipment offshore in this way can be very time consuming and consequently very expensive, and it is also subject to the constraints and hazards of ship borne assembly and test.
An alternative method is to assemble the equipment onshore and use suitable vessels to tow the assembled equipment through the sea to the relevant location. Since the assembled equipment can be checked and verified onshore, towing the preassembled equipment potentially offers a quicker, less hazardous, and less expensive method than assembly on board a vessel. However, towing through the sea at or near the surface results in constant flexing and fatigue of the equipment throughout the tow due to wave action. This results in a reduction in the usable fatigue life of the equipment following installation.
The fatigue problem can be alleviated by towing the equipment at a depth below the wave-affected zone. The equipment can be made to be substantially neutrally buoyant and towed at the required depth through the addition of buoyancy and/or chains suitably spaced along its length. Before installation of the towed equipment, it is recovered to the surface to allow the attached buoyancy and/or chains to be removed. This can be time consuming, costly, and vulnerable to adverse weather conditions. Furthermore, during removal there is a risk that the attachments could fall and damage subsea equipment already in place.